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Abstract:

Disclosed is an autostereoscopic display apparatus comprising a light
guiding valve apparatus including an imaging directional backlight, an
illuminator array and an observer tracking system arranged to achieve
control of an array of illuminators which may provide a directional
display to an observer over a wide lateral and longitudinal viewing range
with low flicker.

Claims:

1. An autostereoscopic display apparatus comprising: a display device
comprising: a transmissive spatial light modulator comprising an array of
pixels arranged to modulate light passing therethrough; a waveguide
having an input end and first and second, opposed guide surfaces for
guiding light along the waveguide that extend from the input end across
the spatial light modulator; and an array of light sources at different
input positions in a lateral direction across the input end of the
waveguide, the waveguide being arranged to direct input light from light
sources at the different input positions across the input end as output
light through the first guide surface for supply through the spatial
light modulator into respective optical windows in output directions
distributed in the lateral direction in dependence on the input positions
the autostereoscopic display apparatus further comprising: a sensor
system arranged to detect the position of an observer relative to the
display device; and a control system arranged to control the spatial
light modulator and the light sources, wherein the control system is
arranged to control the spatial light modulator to modulate light with
left and right images temporally multiplexed in left and right image
phases that alternate with each other, and the control system is arranged
to operate the light sources in the left and right image phases,
selectively to direct the left and right images into respective viewing
windows comprising at least one optical window in positions corresponding
to left and right eyes of an observer, in dependence on the detected
position of the observer, the control system being arranged, when the
position of the viewing windows is static, to operate individual light
sources over a single phase so that the time-average of luminous flux has
a predetermined value, and the control system being arranged, when
shifting the position of the viewing windows in response to the detected
position of the observer changing, to control light sources corresponding
to optical windows of left and right viewing windows that are closest to
each other by ceasing operation of a given light source in one of the
left and right image phases and starting operation of the same or
different light source in the other one of the left and right image
phases, in a manner in which, over each adjacent pair of a left image
phase and a right image phase, the time-average of the luminous flux of
said given light source and the luminous flux of said same or different
light source is more than zero and less than twice said predetermined
value.

2. An autostereoscopic display apparatus according to claim 1, wherein
the control system is arranged, when shifting the position of the viewing
windows in response to the detected position of the observer changing, to
control the operation of said given light source in its final instance of
operation in said one of the left and right image phases so that over
that phase the time-average of luminous flux is less than the
predetermined value and to control the operation of said same or
different light source in its initial instance of operation of said other
of the left and right image phases so that over that phase the
time-average of luminous flux is the predetermined value.

3. An autostereoscopic display apparatus according to claim 1, wherein
the control system is arranged, when shifting the position of the viewing
windows in response to the detected position of the observer changing, to
control the operation of said given light source in its final instance of
operation of said one of the left and right image phases so that over
that phase the time-average of luminous flux is the predetermined value
and to control the operation of said same or different light source in
its initial instance of operation of said other of the left and right
image phases so that over that phase the time-average of luminous flux is
less than the predetermined value.

4. An autostereoscopic display apparatus according to claim 1, wherein
the control system is arranged, when shifting the position of the viewing
windows in response to the detected position of the observer changing, to
control the operation of said given light source in its final instance of
operation of said one of the left and right image phases so that over
that phase the time-average of luminous flux is less than the
predetermined value, and to control the operation of said same or
different light source in its initial instance of operation of said other
of the left and right image phases so that over that phase the
time-average of luminous flux is less than the predetermined value.

5. An autostereoscopic display apparatus according to claim 4, wherein
said final instance of operation of said one of the left and right image
phases of said given light source is before said initial instance of
operation of said other of the left and right image phases of said same
or different light source.

6. An autostereoscopic display apparatus according to claim 4, wherein
said final instance of operation of said one of the left and right image
phases of said given light source is after said initial instance of
operation of said other of the left and right image phases of said same
or different light source and each of the time integral of luminous flux
over the final instance of operation of said one of the left and right
image phases and the time-average of luminous flux over the initial
instance of operation of said other of the left and right image phases is
less than half the predetermined value.

7. An autostereoscopic display apparatus according to claim 1, wherein
the light sources operated in the left image phases are contiguous in the
array of light sources with the light sources operated in the right image
phases so that said same or different light source is the same light
source as said given light source.

8. An autostereoscopic display apparatus according to claim 1, wherein
the light sources operated in the left image phases are separated in the
array of light sources with the light sources operated in the right image
phases so that said same or different light source is a different light
source from said given light source.

9. An autostereoscopic display apparatus according to claim 1, wherein
the first guide surface is arranged to guide light by total internal
reflection and the second guide surface comprises a plurality of light
extraction features oriented to reflect light guided through the
waveguide in directions allowing exit through the first guide surface as
the output light.

10. An autostereoscopic display apparatus according to claim 9, wherein
the second guide surface has intermediate regions between the light
extraction features that are arranged to direct light through the
waveguide without extracting it.

11. An autostereoscopic display apparatus according to claim 10, wherein
the second guide surface has a stepped shape comprising facets, that
constitute said light extraction features, and said intermediate regions.

12. An autostereoscopic display device according to claim 1, wherein the
first guide surface is arranged to guide light by total internal
reflection and the second guide surface is substantially planar and
inclined at an angle to reflect light in directions that break the total
internal reflection for outputting light through the first guide surface,
the display device further comprising a deflection element extending
across the first guide surface of the waveguide for deflecting light
towards the normal to the spatial light modulator.

13. An autostereoscopic display apparatus according to claim 1, wherein
the waveguide has a reflective end facing the input end for reflecting
light from the input light back through the waveguide, the waveguide
being arranged to output light through the first guide surface after
reflection from the reflective end.

14. An autostereoscopic display apparatus according claim 13, wherein the
reflective end has positive optical power in the lateral direction.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent
Application No. 61/649,050, entitled "Control System for a directional
light source," filed May 18, 2012, which is herein incorporated by
reference in its entirety.

TECHNICAL FIELD

[0002] This disclosure generally relates to illumination of light
modulation devices, and more specifically relates to light guides for
providing large area illumination from localized light sources for use in
2D, 3D, and/or autostereoscopic display devices.

BACKGROUND

[0003] Spatially multiplexed autostereoscopic displays typically align a
parallax component such as a lenticular screen or parallax barrier with
an array of images arranged as at least first and second sets of pixels
on a spatial light modulator, for example an LCD. The parallax component
directs light from each of the sets of pixels into different respective
directions to provide first and second viewing windows in front of the
display. An observer with an eye placed in the first viewing window can
see a first image with light from the first set of pixels; and with an
eye placed in the second viewing window can see a second image, with
light from the second set of pixels.

[0004] Such displays have reduced spatial resolution compared to the
native resolution of the spatial light modulator and further, the
structure of the viewing windows is determined by the pixel aperture
shape and parallax component imaging function. Gaps between the pixels,
for example for electrodes, typically produce non-uniform viewing
windows. Undesirably such displays exhibit image flicker as an observer
moves laterally with respect to the display and so limit the viewing
freedom of the display. Such flicker can be reduced by defocusing the
optical elements; however such defocusing results in increased levels of
image cross talk and increases visual strain for an observer. Such
flicker can be reduced by adjusting the shape of the pixel aperture,
however such changes can reduce display brightness and can comprise
addressing electronics in the spatial light modulator.

BRIEF SUMMARY

[0005] According to an aspect of the present disclosure, there may be
provided an autostereoscopic display apparatus which may include a
display device including a transmissive spatial light modulator. The
transmissive spatial light modulator may include an array of pixels
arranged to modulate light passing therethrough. The display device may
also include a waveguide having an input end and first and second opposed
guide surfaces for guiding light along the waveguide. The first and
second opposed guide surfaces may extend from the input end across the
spatial light modulator. The display device may also include an array of
light sources at different input positions in a lateral direction across
the input end of the waveguide. The waveguide may be arranged to direct
input light, from light sources at the different input positions across
the input end, as output light through the first guide surface for supply
through the spatial light modulator into respective optical windows in
output directions distributed in the lateral direction in dependence on
the input positions. The autostereoscopic display apparatus may also
include a sensor system which may be arranged to detect the position of
an observer relative to the display device and a control system which may
be arranged to control the spatial light modulator and the light sources.
The control system may be arranged to control the spatial light modulator
to modulate light with left and right images temporally multiplexed in
left and right image phases that alternate with each other. Additionally,
the control system may be arranged to operate the light sources in the
left and right image phases, selectively to direct the left and right
images into viewing windows which may include at least one optical window
in positions corresponding to left and right eyes of an observer, in
dependence on the detected position of the observer. The control system
may be arranged, when the position of the viewing windows is
substantially static, to operate individual light sources over a single
phase so that the time-average of luminous flux has a predetermined
value. The control system may be arranged, when shifting the position of
the viewing windows in response to the detected position of the observer
changing, to control light sources corresponding to optical windows of
left and right viewing windows that are closest to each other by ceasing
operation of a given light source in one of the left and right image
phases and starting operation of the same or different light source in
the other one of the left and right image phases. This may take place in
a manner in which, over each adjacent pair of a left image phase and a
right image phase, the time-average of the luminous flux of the given
light source and the luminous flux of the same or different light source
may be more than zero and less than twice the predetermined value.

[0006] By processing the waveforms to the LEDs of the light emitting
element illuminator array in the transition regions between left and
right phases the conditions that may result in a brightness artifact can
be compensated for during observer tracking.

[0007] Modifying the LED drive waveforms as described herein may reduce
the appearance of a brightness flicker effect for the observer and thus
improve the quality of the display for a tracked observer. Further, a
waveform may be modified so that the inserted pulse may be arranged
temporally approximately equidistant between preceding and following
pulses. Such an embodiment may achieve further reduction in appearance of
the flicker artifact for a moving observer.

[0008] Display backlights in general employ waveguides and edge emitting
sources. Certain imaging directional backlights have the additional
capability of directing the illumination through a display panel into
viewing windows. An imaging system may be formed between multiple sources
and the respective window images. One example of an imaging directional
backlight is an optical valve that may employ a folded optical system and
hence may also be an example of a folded imaging directional backlight.
Light may propagate substantially without loss in one direction through
the optical valve while counter-propagating light may be extracted by
reflection off tilted facets as described in patent application Ser. No.
13/300,293, which is herein incorporated by reference, in its entirety.

[0009] U.S. Pat. No. 6,377,295, which is herein incorporated by reference
in its entirety, generally discusses that prediction can be used to
correct coordinates due to latency in tracking control. This is applied
to a mechanically moved parallax optical element, the position of which
must be controlled at all times or continuously. By way of comparison the
present embodiments provide a predictive generation of the observer
location, rather than the tracker latency, at a defined future time set
by the display illumination pulses. Advantageously it may not be
appropriate to determine locations continuously, but instead at discrete
future times of the illumination. U.S. Pat. No. 5,959,664, which is
herein incorporated by reference in its entirety, generally discusses
longitudinal tracking of an observer and steering by adjusting the
content of the display SLM. By way of comparison embodiments described
below may achieve longitudinal tracking by adjusting the illumination of
the optical valve without adjusting or slicing of the image on the
display SLM.

[0010] Embodiments herein may provide an autostereoscopic display with
large area and thin structure. Further, as will be described, the optical
valves of the present disclosure may achieve thin optical components with
large back working distances. Such components can be used in directional
backlights, to provide directional displays including autostereoscopic
displays. Further, embodiments may provide a controlled illuminator for
the purposes of an efficient autostereoscopic display.

[0011] Embodiments of the present disclosure may be used in a variety of
optical systems. The embodiment may include or work with a variety of
projectors, projection systems, optical components, displays,
microdisplays, computer systems, processors, self-contained projector
systems, visual and/or audiovisual systems and electrical and/or optical
devices. Aspects of the present disclosure may be used with practically
any apparatus related to optical and electrical devices, optical systems,
presentation systems or any apparatus that may contain any type of
optical system. Accordingly, embodiments of the present disclosure may be
employed in optical systems, devices used in visual and/or optical
presentations, visual peripherals and so on and in a number of computing
environments.

[0012] Before proceeding to the disclosed embodiments in detail, it should
be understood that the disclosure is not limited in its application or
creation to the details of the particular arrangements shown, because the
disclosure is capable of other embodiments. Moreover, aspects of the
disclosure may be set forth in different combinations and arrangements to
define embodiments unique in their own right. Also, the terminology used
herein is for the purpose of description and not of limitation.

[0013] Directional backlights offer control over the illumination
emanating from substantially the entire output surface controlled
typically through modulation of independent LED light sources arranged at
the input aperture side of an optical waveguide. Controlling the emitted
light directional distribution can achieve single person viewing for a
security function, where the display can only be seen by a single viewer
from a limited range of angles; high electrical efficiency, where
illumination is only provided over a small angular directional
distribution; alternating left and right eye viewing for time sequential
stereoscopic and autostereoscopic display; and low cost.

[0014] These and other advantages and features of the present disclosure
will become apparent to those of ordinary skill in the art upon reading
this disclosure in its entirety.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Embodiments are illustrated by way of example in the accompanying
FIGURES, in which like reference numbers indicate similar parts, and in
which:

[0016] FIG. 1A is a schematic diagram illustrating a front view of light
propagation in one embodiment of a directional display device;

[0017] FIG. 1B is a schematic diagram illustrating a side view of light
propagation in one embodiment of the directional display device of FIG.
1A;

[0018] FIG. 2A is a schematic diagram illustrating in a top view of light
propagation in another embodiment of a directional display device;

[0019] FIG. 2B is a schematic diagram illustrating light propagation in a
front view of the directional display device of FIG. 2A;

[0020] FIG. 2C is a schematic diagram illustrating light propagation in a
side view of the directional display device of FIG. 2A;

[0021] FIG. 3 is a schematic diagram illustrating in a side view of a
directional display device;

[0022] FIG. 4A is schematic diagram illustrating in a front view,
generation of a viewing window in a directional display device and
including curved light extraction features;

[0023] FIG. 4B is a schematic diagram illustrating in a front view,
generation of a first and a second viewing window in a directional
display device and including curved light extraction features;

[0024] FIG. 5 is a schematic diagram illustrating generation of a first
viewing window in a directional display device including linear light
extraction features;

[0025] FIG. 6A is a schematic diagram illustrating one embodiment of the
generation of a first viewing window in a time multiplexed imaging
directional display device in a first time slot;

[0026] FIG. 6B is a schematic diagram illustrating another embodiment of
the generation of a second viewing window in a time multiplexed
directional display device in a second time slot;

[0027] FIG. 6C is a schematic diagram illustrating another embodiment of
the generation of a first and a second viewing window in a time
multiplexed directional display device;

[0045] FIG. 21A is a schematic diagram illustrating a full illumination
pulse suitable for a weak emitter;

[0046] FIG. 21B is a schematic diagram illustrating a chopped illumination
pulse suitable for a higher power light emitting element;

[0047] FIG. 21C is a schematic diagram illustrating a shorter pulse
suitable for a higher power light emitting element;

[0048] FIG. 21D is a schematic diagram illustrating a shorter pulse
suitable for a higher power light emitting element;

[0049] FIG. 22 is a schematic diagram illustrating a phased pulse position
modulation;

[0050] FIG. 23 is a schematic diagram illustrating a further embodiment of
a directional display device including an integrated infrared emitting
element array arranged to provide a fan of beams to illuminate an
observer and to provide an observer tracking function;

[0051] FIG. 24 is a schematic timing diagram of illumination pulses
including the double pulse that occurs in one case for a tracked
observer;

[0052] FIG. 25 is a schematic timing diagram of illumination pulses
including the missing pulse that occurs in one case for a tracked
observer;

[0053] FIGS. 26 to 31 are schematic timing diagram of illumination pulses
illustrating further embodiments that reduce the visual effect of the
double pulse;

[0054] FIG. 32 is a schematic diagram illustrating a control method to
drive a switch LED with a modified output to reduce image flicker;

[0055] FIG. 33 is a schematic timing diagram for switching of an LED array
in a first pattern between first and second illumination positions;

[0056] FIG. 34 is a schematic timing diagram for switching of an LED array
in a second pattern between first and second illumination positions;

[0057] FIG. 35 is a schematic diagram illustrating a control system to
achieve uniform switching of an LED array between first and second
positions;

[0058] FIG. 36 is a schematic diagram illustrating switching of an LED
array in a second pattern between first and second illumination positions
with no central LED control;

[0059] FIG. 37 is a schematic diagram illustrating switching of an LED
array in a second pattern between first and second illumination positions
with no central LED control;

[0060] FIGS. 38 to 42 are schematic timing diagram of illumination pulses
that reduce the visual effect of a double pulse;

[0061] FIG. 43 is a schematic diagram illustrating a control method to
identify switching LED groups;

[0062] FIG. 44 is a schematic diagram illustrating a further control
method to identify switching LED groups in a first illumination
arrangement; and

[0063] FIG. 45 is a schematic diagram illustrating a further control
method to identify switching LED groups in a second illumination
arrangement.

DETAILED DESCRIPTION

[0064] Time multiplexed autostereoscopic displays can advantageously
improve the spatial resolution of autostereoscopic display by directing
light from all of the pixels of a spatial light modulator to a first
viewing window in a first time slot, and all of the pixels to a second
viewing window in a second time slot. Thus an observer with eyes arranged
to receive light in first and second viewing windows will see a full
resolution image across the whole of the display over multiple time
slots. Time multiplexed displays can advantageously achieve directional
illumination by directing an illuminator array through a substantially
transparent time multiplexed spatial light modulator using directional
optical elements, wherein the directional optical elements substantially
form an image of the illuminator array in the window plane.

[0065] The uniformity of the viewing windows may be advantageously
independent of the arrangement of pixels in the spatial light modulator.
Advantageously, such displays can provide observer tracking displays
which have low flicker, with low levels of cross talk for a moving
observer.

[0066] To achieve high uniformity in the window plane, it is desirable to
provide an array of illumination elements that have a high spatial
uniformity. The illuminator elements of the time sequential illumination
system may be provided, for example, by pixels of a spatial light
modulator with size approximately 100 micrometers in combination with a
lens array. However, such pixels suffer from similar difficulties as for
spatially multiplexed displays. Further, such devices may have low
efficiency and higher cost, requiring additional display components.

[0067] High window plane uniformity can be conveniently achieved with
macroscopic illuminators, for example, an array of LEDs in combination
with homogenizing and diffusing optical elements that are typically of
size 1 mm or greater. However, the increased size of the illuminator
elements means that the size of the directional optical elements
increases proportionately. For example, a 16 mm wide illuminator imaged
to a 65 mm wide viewing window may require a 200 mm back working
distance. Thus, the increased thickness of the optical elements can
prevent useful application, for example, to mobile displays, or large
area displays.

[0068] Addressing the aforementioned shortcomings, optical valves as
described in commonly-owned U.S. patent application Ser. No. 13/300,293
advantageously can be arranged in combination with fast switching
transmissive spatial light modulators to achieve time multiplexed
autostereoscopic illumination in a thin package while providing high
resolution images with flicker free observer tracking and low levels of
cross talk. Described is a one dimensional array of viewing positions, or
windows, that can display different images in a first, typically
horizontal, direction, but contain the same images when moving in a
second, typically vertical, direction.

[0069] Conventional non-imaging display backlights commonly employ optical
waveguides and have edge illumination from light sources such as LEDs.
However, it should be appreciated that there are many fundamental
differences in the function, design, structure, and operation between
such conventional non-imaging display backlights and the imaging
directional backlights discussed in the present disclosure.

[0070] Generally, for example, in accordance with the present disclosure,
imaging directional backlights are arranged to direct the illumination
from multiple light sources through a display panel to respective
multiple viewing windows in at least one axis. Each viewing window is
substantially formed as an image in at least one axis of a light source
by the imaging system of the imaging directional backlight. An imaging
system may be formed between multiple light sources and the respective
window images. In this manner, the light from each of the multiple light
sources is substantially not visible for an observer's eye outside of the
respective viewing window.

[0071] In contradistinction, conventional non-imaging backlights or light
guiding plates (LGPs) are used for illumination of 2D displays. See,
e.g., Kalil Kalantar et al., Backlight Unit With Double Surface Light
Emission, J. Soc. Inf. Display, Vol. 12, Issue 4, pp. 379-387 (December
2004). Non-imaging backlights are typically arranged to direct the
illumination from multiple light sources through a display panel into a
substantially common viewing zone for each of the multiple light sources
to achieve wide viewing angle and high display uniformity. Thus
non-imaging backlights do not form viewing windows. In this manner, the
light from each of the multiple light sources may be visible for an
observer's eye at substantially all positions across the viewing zone.
Such conventional non-imaging backlights may have some directionality,
for example, to increase screen gain compared to Lambertian illumination,
which may be provided by brightness enhancement films such as BEF®
from 3M. However, such directionality may be substantially the same for
each of the respective light sources. Thus, for these reasons and others
that should be apparent to persons of ordinary skill, conventional
non-imaging backlights are different to imaging directional backlights.
Edge lit non-imaging backlight illumination structures may be used in
liquid crystal display systems such as those seen in 2D Laptops, Monitors
and TVs. Light propagates from the edge of a lossy waveguide which may
include sparse features; typically local indentations in the surface of
the guide which cause light to be lost regardless of the propagation
direction of the light.

[0072] As used herein, an optical valve is an optical structure that may
be a type of light guiding structure or device referred to as, for
example, a light valve, an optical valve directional backlight, and a
valve directional backlight ("v-DBL"). In the present disclosure, optical
valve is different to a spatial light modulator (which is sometimes
referred to as a "light valve"). One example of an imaging directional
backlight is an optical valve that may employ a folded optical system.
Light may propagate substantially without loss in one direction through
the optical valve, may be incident on an imaging reflector, and may
counter-propagate such that the light may be extracted by reflection off
tilted light extraction features, and directed to viewing windows as
described in U.S. patent application Ser. No. 13/300,293, which is herein
incorporated by reference in its entirety.

[0073] As used herein, examples of an imaging directional backlight
include a stepped waveguide imaging directional backlight, a folded
imaging directional backlight, a wedge type directional backlight, or an
optical valve.

[0074] Additionally, as used herein, a stepped waveguide imaging
directional backlight may be an optical valve. A stepped waveguide is a
waveguide for an imaging directional backlight comprising a waveguide for
guiding light, which may include a first light guiding surface and a
second light guiding surface, opposite the first light guiding surface,
further comprising a plurality of light guiding features interspersed
with a plurality of extraction features arranged as steps.

[0075] Moreover, as used, a folded imaging directional backlight may be at
least one of a wedge type directional backlight, or an optical valve.

[0076] In operation, light may propagate within an exemplary optical valve
in a first direction from an input end to a reflective end and may be
transmitted substantially without loss. Light may be reflected at the
reflective end and propagates in a second direction substantially
opposite the first direction. As the light propagates in the second
direction, the light may be incident on light extraction features, which
are operable to redirect the light outside the optical valve. Stated
differently, the optical valve generally allows light to propagate in the
first direction and may allow light to be extracted while propagating in
the second direction.

[0077] The optical valve may achieve time sequential directional
illumination of large display areas. Additionally, optical elements may
be employed that are thinner than the back working distance of the
optical elements to direct light from macroscopic illuminators to a
nominal window plane. Such displays may use an array of light extraction
features arranged to extract light counter propagating in a substantially
parallel waveguide.

[0078] Thin imaging directional backlight implementations for use with
LCDs have been proposed and demonstrated by 3M, for example U.S. Pat. No.
7,528,893; by Microsoft, for example U.S. Pat. No. 7,970,246 which may be
referred to herein as a "wedge type directional backlight;" by RealD, for
example U.S. patent application Ser. No. 13/300,293 which may be referred
to herein as an "optical valve" or "optical valve directional backlight,"
all of which are herein incorporated by reference in their entirety.

[0079] The present disclosure provides stepped waveguide imaging
directional backlights in which light may reflect back and forth between
the internal faces of, for example, a stepped waveguide which may include
a first side and a first set of features. As the light travels along the
length of the stepped waveguide, the light may not substantially change
angle of incidence with respect to the first side and first set of
surfaces and so may not reach the critical angle of the medium at these
internal faces. Light extraction may be advantageously achieved by a
second set of surfaces (the step "risers") that are inclined to the first
set of surfaces (the step "treads"). Note that the second set of surfaces
may not be part of the light guiding operation of the stepped waveguide,
but may be arranged to provide light extraction from the structure. By
contrast, a wedge type imaging directional backlight may allow light to
guide within a wedge profiled waveguide having continuous internal
surfaces. The optical valve is thus not a wedge type imaging directional
backlight.

[0080] FIG. 1A is a schematic diagram illustrating a front view of light
propagation in one embodiment of a directional display device, and FIG.
1B is a schematic diagram illustrating a side view of light propagation
in the optical valve structure of FIG. 1A.

[0081] FIG. 1A illustrates a front view in the xy plane of a directional
backlight of a directional display device, and includes an illuminator
array 15 which may be used to illuminate a stepped waveguide 1.
Illuminator array 15 includes illuminator elements 15a through
illuminator element 15n (where n is an integer greater than one). In one
example, the stepped waveguide 1 of FIG. 1A may be a stepped, display
sized waveguide 1. Illuminator elements 15a through 15n are light sources
that may be light emitting diodes (LEDs). Although LEDs are discussed
herein as illuminator elements 15a-15n, other light sources may be used
such as, but not limited to, diode sources, semiconductor sources, laser
sources, local field emission sources, organic emitter arrays, and so
forth. Additionally, FIG. 1B illustrates a side view in the xz plane, and
includes illuminator array 15, SLM (spatial light modulator) 48,
extraction features 12, guiding features 10, and stepped waveguide 1,
arranged as shown. The side view provided in FIG. 1B is an alternative
view of the front view shown in FIG. 1A. Accordingly, the illuminator
array 15 of FIGS. 1A and 1B corresponds to one another and the stepped
waveguide 1 of FIGS. 1A and 1B may correspond to one another.

[0082] Further, in FIG. 1B, the stepped waveguide 1 may have an input end
2 that is thin and a reflective end 4 that is thick. Thus the waveguide 1
extends between the input end 2 that receives input light and the
reflective end 4 that reflects the input light back through the waveguide
1. The length of the input end 2 in a lateral direction across the
waveguide is greater than the height of the input end 2. The illuminator
elements 15a-15n are disposed at different input positions in a lateral
direction across the input end 2.

[0083] The waveguide 1 has first and second, opposed guide surfaces
extending between the input end 2 and the reflective end 4 for guiding
light forwards and back along the waveguide 1 by total internal
reflection. The first guide surface is planar. The second guide surface
has a plurality of light extraction features 12 facing the reflective end
4 and inclined to reflect at least some of the light guided back through
the waveguide 1 from the reflective end in directions that break the
total internal reflection at the first guide surface and allow output
through the first guide surface, for example, upwards in FIG. 1B, that is
supplied to the SLM 48.

[0084] In this example, the light extraction features 12 are reflective
facets, although other reflective features could be used. The light
extraction features 12 do not guide light through the waveguide, whereas
the intermediate regions of the second guide surface intermediate the
light extraction features 12 guide light without extracting it. Those
regions of the second guide surface are planar and may extend parallel to
the first guide surface, or at a relatively low inclination. The light
extraction features 12 extend laterally to those regions so that the
second guide surface has a stepped shape including the light extraction
features 12 and intermediate regions. The light extraction features 12
are oriented to reflect light from the light sources, after reflection
from the reflective end 4, through the first guide surface.

[0085] The light extraction features 12 are arranged to direct input light
from different input positions in the lateral direction across the input
end in different directions relative to the first guide surface that are
dependent on the input position. As the illumination elements 15a-15n are
arranged at different input positions, the light from respective
illumination elements 15a-15n is reflected in those different directions.
In this manner, each of the illumination elements 15a-15n directs light
into a respective optical window in output directions distributed in the
lateral direction in dependence on the input positions. The lateral
direction across the input end 2 in which the input positions are
distributed corresponds with regard to the output light to a lateral
direction to the normal to the first guide surface. The lateral
directions as defined at the input end 2 and with regard to the output
light remain parallel in this embodiment where the deflections at the
reflective end 4 and the first guide surface are generally orthogonal to
the lateral direction. Under the control of a control system, the
illuminator elements 15a-15n may be selectively operated to direct light
into a selectable optical window.

[0086] In the present disclosure an optical window may correspond to the
image of a single light source in the window plane, being a nominal plane
in which optical windows form across the entirety of the display device.
Alternatively, an optical windows may correspond to the image of a groups
of light sources that are driven together. Advantageously, such groups of
light sources may increase uniformity of the optical windows of the array
121.

[0087] By way of comparison, a viewing window is a region in the window
plane wherein light is provided comprising image data of substantially
the same image from across the display area. Thus a viewing window may be
formed from a single optical window or from plural optical windows.

[0088] The SLM 48 extends across the waveguide is transmissive and
modulates the light passing therethrough. Although the SLM 48 may be a
liquid crystal display (LCD) but this is merely by way of example, and
other spatial light modulators or displays may be used including LCOS,
DLP devices, and so forth, as this illuminator may work in reflection. In
this example, the SLM 48 is disposed across the first guide surface of
the waveguide and modulates the light output through the first guide
surface after reflection from the light extraction features 12.

[0089] The operation of a directional display device that may provide a
one dimensional array of viewing windows is illustrated in front view in
FIG. 1A, with its side profile shown in FIG. 1B. In operation, in FIGS.
1A and 1B, light may be emitted from an illuminator array 15, such as an
array of illuminator elements 15a through 15n, located at different
positions, y, along the surface of thin end side 2, x=0, of the stepped
waveguide 1. The light may propagate along +x in a first direction,
within the stepped waveguide 1, while at the same time, the light may fan
out in the xy plane and upon reaching the far curved end side 4, may
substantially or entirely fill the curved end side 4. While propagating,
the light may spread out to a set of angles in the xz plane up to, but
not exceeding the critical angle of the guide material. The extraction
features 12 that link the guiding features 10 of the bottom side of the
stepped waveguide 1 may have a tilt angle greater than the critical angle
and hence may be missed by substantially all light propagating along +x
in the first direction, ensuring the substantially lossless forward
propagation.

[0090] Continuing the discussion of FIGS. 1A and 1B, the curved end side 4
of the stepped waveguide 1 may be made reflective, typically by being
coated with a reflective material such as, for example, silver, although
other reflective techniques may be employed. Light may therefore be
redirected in a second direction, back down the guide in the direction of
-x and may be substantially collimated in the xy or display plane. The
angular spread may be substantially preserved in the xz plane about the
principal propagation direction, which may allow light to hit the riser
edges and reflect out of the guide. In an embodiment with approximately
45 degree tilted extraction features 12, light may be effectively
directed approximately normal to the xy display plane with the xz angular
spread substantially maintained relative to the propagation direction.
This angular spread may be increased when light exits the stepped
waveguide 1 through refraction, but may be decreased somewhat dependent
on the reflective properties of the extraction features 12.

[0091] In some embodiments with uncoated extraction features 12,
reflection may be reduced when total internal reflection (TIR) fails,
squeezing the xz angular profile and shifting off normal. However, in
other embodiments having silver coated or metallized extraction features,
the increased angular spread and central normal direction may be
preserved. Continuing the description of the embodiment with silver
coated extraction features, in the xz plane, light may exit the stepped
waveguide 1 approximately collimated and may be directed off normal in
proportion to the y-position of the respective illuminator element
15a-15n in illuminator array 15 from the input edge center. Having
independent illuminator elements 15a-15n along the input edge 2 then
enables light to exit from the entire first light directing side 6 and
propagate at different external angles, as illustrated in FIG. 1A.

[0092] In one embodiment, a display device may include a stepped waveguide
or light valve which in turn, may include a first guide surface that may
be arranged to guide light by total internal reflection. The light valve
may include a second guide surface which may have a plurality of light
extraction features inclined to reflect light guided through the
waveguide in directions allowing exit through the first guide surface as
the output light. The second guide surface may also have regions between
the light extraction features that may be arranged to direct light
through the waveguide without extracting it.

[0093] In another embodiment, a display device may include a waveguide
with at least a first guide surface which may be arranged to guide light
by total internal reflection and a second guide surface which may be
substantially planar and inclined at an angle to reflect light in
directions that break the total internal reflection for outputting light
through the first guide surface, The display device may include a
deflection element extending across the first guide surface of the
waveguide for deflecting light towards the normal to the SLM 48.

[0094] In yet another embodiment, a display device may include a waveguide
which may have a reflective end facing the input end for reflecting light
from the input light back through the waveguide. The waveguide may
further be arranged to output light through the first guide surface after
reflection from the reflective end.

[0095] Illuminating an SLM 48 such as a fast liquid crystal display (LCD)
panel with such a device may achieve autostereoscopic 3D as shown in top
view or yz-plane viewed from the illuminator array 15 end in FIG. 2A,
front view in FIG. 2B and side view in FIG. 2C. FIG. 2A is a schematic
diagram illustrating in a top view, propagation of light in a directional
display device, FIG. 2B is a schematic diagram illustrating in a front
view, propagation of light in a directional display device, and FIG. 2C
is a schematic diagram illustrating in side view propagation of light in
a directional display device. As illustrated in FIGS. 2A, 2B, and 2C, a
stepped waveguide 1 may be located behind a fast (e.g., greater than 100
Hz) LCD panel SLM 48 that displays sequential right and left eye images.
In synchronization, specific illuminator elements 15a through 15n of
illuminator array 15 (where n is an integer greater than one) may be
selectively turned on and off, providing illuminating light that enters
right and left eyes substantially independently by virtue of the system's
directionality. In the simplest case, sets of illuminator elements of
illuminator array 15 are turned on together, providing a one dimensional
viewing window 26 or an optical pupil with limited width in the
horizontal direction, but extended in the vertical direction, in which
both eyes horizontally separated may view a left eye image, and another
viewing window 44 in which a right eye image may primarily be viewed by
both eyes, and a central position in which both the eyes may view
different images. In this way, 3D may be viewed when the head of a viewer
is approximately centrally aligned. Movement to the side away from the
central position may result in the scene collapsing onto a 2D image.

[0096] The reflective end 4 may have positive optical power in the lateral
direction across the waveguide. In embodiments in which typically the
reflective end 4 has positive optical power, the optical axis may be
defined with reference to the shape of the reflective end 4, for example
being a line that passes through the centre of curvature of the
reflective end 4 and coincides with the axis of reflective symmetry of
the end 4 about the x-axis. In the case that the reflecting surface 4 is
flat, the optical axis may be similarly defined with respect to other
components having optical power, for example the light extraction
features 12 if they are curved, or the Fresnel lens 62 described below.
The optical axis 238 is typically coincident with the mechanical axis of
the waveguide 1. In the present embodiments that typically comprise a
substantially cylindrical reflecting surface at end 4, the optical axis
238 is a line that passes through the centre of curvature of the surface
at end 4 and coincides with the axis of reflective symmetry of the side 4
about the x-axis. The optical axis 238 is typically coincident with the
mechanical axis of the waveguide 1. The cylindrical reflecting surface at
end 4 may typically comprise a spherical profile to optimize performance
for on-axis and off-axis viewing positions. Other profiles may be used.

[0097] FIG. 3 is a schematic diagram illustrating in side view a
directional display device. Further, FIG. 3 illustrates additional detail
of a side view of the operation of a stepped waveguide 1, which may be a
transparent material. The stepped waveguide 1 may include an illuminator
input end 2, a reflective end 4, a first light directing side 6 which may
be substantially planar, and a second light directing side 8 which
includes guiding features 10 and light extraction features 12. In
operation, light rays 16 from an illuminator element 15c of an
illuminator array 15 (not shown in FIG. 3), that may be an addressable
array of LEDs for example, may be guided in the stepped waveguide 1 by
means of total internal reflection by the first light directing side 6
and total internal reflection by the guiding feature 10, to the
reflective end 4, which may be a mirrored surface. Although reflective
end 4 may be a mirrored surface and may reflect light, it may in some
embodiments also be possible for light to pass through reflective end 4.

[0098] Continuing the discussion of FIG. 3, light ray 18 reflected by the
reflective end 4 may be further guided in the stepped waveguide 1 by
total internal reflection at the reflective end 4 and may be reflected by
extraction features 12. Light rays 18 that are incident on extraction
features 12 may be substantially deflected away from guiding modes of the
stepped waveguide 1 and may be directed, as shown by ray 20, through the
side 6 to an optical pupil that may form a viewing window 26 of an
autostereoscopic display. The width of the viewing window 26 may be
determined by at least the size of the illuminator, output design
distance and optical power in the side 4 and extraction features 12. The
height of the viewing window may be primarily determined by the
reflection cone angle of the extraction features 12 and the illumination
cone angle input at the input end 2. Thus each viewing window 26
represents a range of separate output directions with respect to the
surface normal direction of the SLM 48 that intersect with a plane at the
nominal viewing distance.

[0099] FIG. 4A is a schematic diagram illustrating in front view a
directional display device which may be illuminated by a first
illuminator element and including curved light extraction features. In
FIG. 4A, the directional backlight may include the stepped waveguide 1
and the light source illuminator array 15. Further, FIG. 4A shows in
front view further guiding of light rays from illuminator element 15c of
illuminator array 15, in the stepped waveguide 1. Each of the output rays
are directed towards the same viewing window 26 from the respective
illuminator 14. Thus light ray 30 may intersect the ray 20 in the window
26, or may have a different height in the window as shown by ray 32.
Additionally, in various embodiments, sides 22, 24 of the waveguide may
be transparent, mirrored, or blackened surfaces. Continuing the
discussion of FIG. 4A, light extraction features 12 may be elongate, and
the orientation of light extraction features 12 in a first region 34 of
the light directing side 8 (light directing side 8 shown in FIG. 3, but
not shown in FIG. 4A) may be different to the orientation of light
extraction features 12 in a second region 36 of the light directing side
8.

[0100] FIG. 4B is a schematic diagram illustrating in front view a
directional display device which may illuminated by a second illuminator
element. Further, FIG. 4B shows the light rays 40, 42 from a second
illuminator element 15h of the illuminator array 15. The curvature of the
reflective surface on the side 4 and the light extraction features 12
cooperatively produce a second viewing window 44 laterally separated from
the viewing window 26 with light rays from the illuminator element 15h.

[0101] Advantageously, the arrangement illustrated in FIG. 4B may provide
a real image of the illuminator element 15c at a viewing window 26 in
which the real image may be formed by cooperation of optical power in
reflective end 4 and optical power which may arise from different
orientations of elongate light extraction features 12 between regions 34
and 36, as shown in FIG. 4A. The arrangement of FIG. 4B may achieve
improved aberrations of the imaging of illuminator element 15c to lateral
positions in viewing window 26. Improved aberrations may achieve an
extended viewing freedom for an autostereoscopic display while achieving
low cross talk levels.

[0102] FIG. 5 is a schematic diagram illustrating in front view an
embodiment of a directional display device comprising a waveguide 1
having substantially linear light extraction features. Further, FIG. 5
shows a similar arrangement of components to FIG. 1 (with corresponding
elements being similar), with one of the differences being that the light
extraction features 12 are substantially linear and parallel to each
other. Advantageously, such an arrangement may provide substantially
uniform illumination across a display surface and may be more convenient
to manufacture than the curved extraction features of FIG. 4A and FIG.
4B.

[0103] FIG. 6A is a schematic diagram illustrating one embodiment of the
generation of a first viewing window in a time multiplexed imaging
directional display device, namely an optical valve apparatus in a first
time slot. FIG. 6B is a schematic diagram illustrating another embodiment
of the generation of a second viewing window in a time multiplexed
imaging directional backlight apparatus in a second time slot. FIG. 6C is
a schematic diagram illustrating another embodiment of the generation of
a first and a second viewing window in a time multiplexed imaging
directional display device. Further, FIG. 6A shows schematically the
generation of illumination window 26 from stepped waveguide 1.
Illuminator element group 31 in illuminator array 15 may provide a light
cone 17 directed towards a viewing window 26. FIG. 6B shows schematically
the generation of illumination window 44. Illuminator element group 33 in
illuminator array 15 may provide a light cone 19 directed towards viewing
window 44. In cooperation with a time multiplexed display, windows 26 and
44 may be provided in sequence as shown in FIG. 6C. If the image on a SLM
48 (not shown in FIGS. 6A, 6B, 6C) is adjusted in correspondence with the
light direction output, then an autostereoscopic image may be achieved
for a suitably placed viewer. Similar operation can be achieved with all
the directional backlights and directional display devices described
herein. Note that illuminator element groups 31, 33 each include one or
more illumination elements from illumination elements 15a to 15n, where n
is an integer greater than one.

[0104] FIG. 7 is a schematic diagram illustrating one embodiment of an
observer tracking autostereoscopic directional display device including a
time multiplexed directional backlight. As shown in FIG. 7, selectively
turning on and off illuminator elements 15a to 15n along axis 29 provides
for directional control of viewing windows. The head 45 position may be
monitored with a camera, motion sensor, motion detector, or any other
appropriate optical, mechanical or electrical means, and the appropriate
illuminator elements of illuminator array 15 may be turned on and off to
provide substantially independent images to each eye irrespective of the
head 45 position. The head tracking system (or a second head tracking
system) may provide monitoring of more than one head 45, 47 (head 47 not
shown in FIG. 7) and may supply the same left and right eye images to
each viewers' left and right eyes providing 3D to all viewers. Again
similar operation can be achieved with all the directional backlights and
directional display devices described herein.

[0105] FIG. 8 is a schematic diagram illustrating one embodiment of a
multi-viewer directional display device as an example including an
imaging directional backlight. As shown in FIG. 8, at least two 2D images
may be directed towards a pair of viewers 45, 47 so that each viewer may
watch a different image on the SLM 48. The two 2D images of FIG. 8 may be
generated in a similar manner as described with respect to FIG. 7 in that
the two images would be displayed in sequence and in synchronization with
sources whose light is directed toward the two viewers. One image is
presented on the SLM 48 in a first phase, and a second image is presented
on the SLM 48 in a second phase different from the first phase. In
correspondence with the first and second phases, the output illumination
is adjusted to provide first and second viewing windows 26, 44
respectively. An observer with both eyes in window 26 will perceive a
first image while an observer with both eyes in window 44 will perceive a
second image.

[0106] FIG. 9 is a schematic diagram illustrating a privacy directional
display device which includes an imaging directional backlight. 2D image
display systems may also utilize directional backlighting for security
and efficiency purposes in which light may be primarily directed at the
eyes of a first viewer 45 as shown in FIG. 9. Further, as illustrated in
FIG. 9, although first viewer 45 may be able to view an image on device
50, light is not directed towards second viewer 47. Thus second viewer 47
is prevented from viewing an image on device 50. Each of the embodiments
of the present disclosure may advantageously provide autostereoscopic,
dual image or privacy display functions.

[0107] FIG. 10 is a schematic diagram illustrating in side view the
structure of a time multiplexed directional display device as an example
including an imaging directional backlight. Further, FIG. 10 shows in
side view an autostereoscopic directional display device, which may
include the stepped waveguide 1 and a Fresnel lens 62 arranged to provide
the viewing window 26 for a substantially collimated output across the
stepped waveguide 1 output surface. A vertical diffuser 68 may be
arranged to extend the height of the window 26 further. The light may
then be imaged through the SLM 48. The illuminator array 15 may include
light emitting diodes (LEDs) that may, for example, be phosphor converted
blue LEDs, or may be separate RGB LEDs. Alternatively, the illuminator
elements in illuminator array 15 may include a uniform light source and
SLM 48 arranged to provide separate illumination regions. Alternatively
the illuminator elements may include laser light source(s). The laser
output may be directed onto a diffuser by means of scanning, for example,
using a galvo or MEMS scanner. In one example, laser light may thus be
used to provide the appropriate illuminator elements in illuminator array
15 to provide a substantially uniform light source with the appropriate
output angle, and further to provide reduction in speckle. Alternatively,
the illuminator array 15 may be an array of laser light emitting
elements. Additionally in one example, the diffuser may be a wavelength
converting phosphor, so that illumination may be at a different
wavelength to the visible output light.

[0108] FIG. 11A is a schematic diagram illustrating a front view of
another imaging directional display device, as illustrated, a wedge type
directional backlight, and FIG. 11B is a schematic diagram illustrating a
side view of the same wedge type directional display device. A wedge type
directional backlight is generally discussed by U.S. Pat. No. 7,660,047
and entitled "Flat Panel Lens," which is herein incorporated by reference
in its entirety. The structure may include a wedge type waveguide 1104
with a bottom surface which may be preferentially coated with a
reflecting layer 1106 and with an end corrugated surface 1102, which may
also be preferentially coated with a reflecting layer 1106. As shown in
FIG. 11B, light may enter the wedge type waveguide 1104 from local
sources 1101 and the light may propagate in a first direction before
reflecting off the end surface. Light may exit the wedge type waveguide
1104 while on its return path and may illuminate a display panel 1110. By
way of comparison with an optical valve, a wedge type waveguide provides
extraction by a taper that reduces the incidence angle of propagating
light so that when the light is incident at the critical angle on an
output surface, it may escape. Escaping light at the critical angle in
the wedge type waveguide propagates substantially parallel to the surface
until deflected by a redirection layer 1108 such as a prism array. Errors
or dust on the wedge type waveguide output surface may change the
critical angle, creating stray light and uniformity errors. Further, an
imaging directional backlight that uses a mirror to fold the beam path in
the wedge type directional backlight may employ a faceted mirror that
biases the light cone directions in the wedge type waveguide. Such
faceted mirrors are generally complex to fabricate and may result in
illumination uniformity errors as well as stray light.

[0109] The wedge type directional backlight and optical valve further
process light beams in different ways. In the wedge type waveguide, light
input at an appropriate angle will output at a defined position on a
major surface, but light rays will exit at substantially the same angle
and substantially parallel to the major surface. By comparison, light
input to a stepped waveguide of an optical valve at a certain angle may
output from points across the first side, with output angle determined by
input angle. Advantageously, the stepped waveguide of the optical valve
may not require further light re-direction films to extract light towards
an observer and angular non-uniformities of input may not provide
non-uniformities across the display surface.

[0110] There follows a description of some directional display apparatuses
including a directional display device and a control system, wherein the
directional display device includes a directional backlight including a
waveguide and an SLM. In the following description, the waveguides,
directional backlights and directional display devices are based on and
incorporate the structures of FIGS. 1 to 11B above. Except for the
modifications and/or additional features which will now be described, the
above description applies equally to the following waveguides,
directional backlights and display devices, but for brevity will not be
repeated.

[0111] FIG. 12 is a schematic diagram illustrating a directional display
apparatus comprising a display device 100 and a control system. The
arrangement and operation of the control system will now be described and
may be applied, mutatis mutandis, to each of the display devices
disclosed herein. As illustrated in FIG. 12, a directional display device
100 may include a directional backlight device that may itself include a
stepped waveguide 1 and a light source illuminator array 15. As
illustrated in FIG. 12, the stepped waveguide 1 includes a light
directing side 8, a reflective end 4, guiding features 10 and light
extraction features 12. The directional display device 100 may further
include an SLM 48.

[0112] The waveguide 1 is arranged as described above. The reflective end
4 converges the reflected light. A Fresnel lens 62 may be arranged to
cooperate with reflective end 4 to achieve viewing windows 26 at a
viewing plane 106 observed by an observer 99. A transmissive SLM 48 may
be arranged to receive the light from the directional backlight. Further
a diffuser 68 may be provided to substantially remove Moire beating
between the waveguide 1 and pixels of the SLM 48 as well as the Fresnel
lens structure62.

[0113] The control system may comprise a sensor system arranged to detect
the position of the observer 99 relative to the display device 100. The
sensor system comprises a position sensor 70, such as a camera, and a
head position measurement system 72 that may for example comprise a
computer vision image processing system. The control system may further
comprise an illumination controller 74 and an image controller 76 that
are both supplied with the detected position of the observer supplied
from the head position measurement system 72.

[0114] The illumination controller 74 selectively operates the illuminator
elements 15 to direct light to into the viewing windows 26 in cooperation
with waveguide 1. The illumination controller 74 selects the illuminator
elements 15 to be operated in dependence on the position of the observer
detected by the head position measurement system 72, so that the viewing
windows 26 into which light is directed are in positions corresponding to
the left and right eyes of the observer 99. In this manner, the lateral
output directionality of the waveguide 1 corresponds with the observer
position.

[0115] The image controller 76 controls the SLM 48 to display images. To
provide an autostereoscopic display, the image controller 76 and the
illumination controller 74 may operate as follows. The image controller
76 controls the SLM 48 to display temporally multiplexed left and right
eye images. The illumination controller 74 operate the light sources 15
to direct light into respective viewing windows in positions
corresponding to the left and right eyes of an observer synchronously
with the display of left and right eye images. In this manner, an
autostereoscopic effect is achieved using a time division multiplexing
technique.

[0116] FIG. 13 is a schematic diagram illustrating in front view, the
formation of viewing windows. Further, FIG. 13 shows in top view, the
embodiment of FIG. 12. Display 100 may produce a fan of light cones 102
and an array of viewing windows 104 in the window plane 106, being a
nominal plane. An observer 99 with nose location 112 may see illumination
from display 100. When a left eye 110 is approximately aligned with
window 116 and a right eye 108 is approximately aligned with window 114
and image data presented in windows 114 and 116 is a stereo pair, then an
autostereoscopic 3D image may be perceived by the observer. The windows
114 and 116 may alternatively show substantially the same data so the
display device 100 may function in as a 2D image display device. The
windows 114 and 116 may be illuminated in separate time slots in
synchronization with the display on the panel of left and right eye image
data.

[0117] There will now be described various arrangements of viewing
windows. Each of these may be provided by appropriate operation of the
control system as described above, for example by selectively operating
the illuminator elements 15 to direct light to into the viewing windows
26 in synchronization with the display of images on the SLM 48. The
directional display apparatus may be operable to provide any one of these
viewing window arrangements, or any combination of these viewing window
arrangements at the same or different times, for example in different
modes of operation of the directional display apparatus.

[0118] In the various drawings illustrating arrangements of viewing
windows, the structure of optical windows illustrates the nominal
position of the optical windows rather than the actual light
distributions which may take a variety of forms and may overlap.

[0119] FIG. 14A is a schematic diagram illustrating in front view, a first
viewing window arrangement. Further, FIG. 14A shows in front view, the
embodiment of FIG. 12. The observer 99 is illustrated as slightly to the
right of a plane 118 normal to the approximate center of the display 100.
Accordingly left and right eye viewing windows 114, 116 may be generated
slightly to the right of the display. In FIG. 14B the observer 99 is
illustrated as being repositioned in direction 120 to the right and so
windows 114, 116 may be steered to the right in response. FIG. 14B is a
schematic diagram illustrating in front view, a second viewing window
arrangement for a moving observer. Advantageously the left and right eyes
of the observer may be illuminated with left and right eye image data
during observer movement.

[0120] Window movement may be provided by mechanical movement of the
illuminator array 15 in correspondence with observer 99 movement in the
window plane 106. However, such movement is complicated and expensive. It
is thus desirable to achieve a reduction in the cost and complexity of
movement of illuminator elements of illuminator array 15 through
switching of discrete illuminator elements, under the control of the
control system.

[0121] FIG. 15 is a schematic diagram illustrating the appearance of
windows of FIG. 14A in the window plane 106 for first and second tracked
observer positions. Further, FIG. 15 shows schematically an array 121 of
optical windows (that may also be referred to as sub-windows) which may
be arranged to achieve a switchable array of viewing windows. Each
optical window of the array 121 may correspond to the image in the window
plane 106 such as shown in FIGS. 12 and 13 of an illuminator element of
the illuminator array 15, as described above.

[0122] The illuminated structure of an optical window array 121 in the
window plane 106 may approximately correspond to the lateral location of
observer 99 as shown in FIG. 14A. In the present embodiment viewing
window 116 for the left eye may include optical window 122 and the
optical window array 134. The right eye viewing window 114 may include
optical window 124 and optical window array 136. Optical windows 125, 126
and 128 may not be illuminated, so that the respective illuminator
elements may not be illuminated.

[0123] Further, FIG. 15 shows the detail of the optical window array 121
approximately corresponding to the location of observer 99 as shown in
FIG. 14B after movement in direction 120. Window boundary 131 is marked
to show the relative position of illuminated optical windows. The left
eye viewing window 116 may be arranged to include optical window 126 and
optical window array 134. Thus, optical window 122 may be turned off.
Similarly for the right eye viewing window, optical window 128 may be
turned on and optical window 124 may be turned off, so that the right eye
viewing window 114 is arranged to include optical window 128 and optical
window array 136. Window 125 remains non-illuminated for the observer
movement shown, reducing cross talk.

[0124] Advantageously such an embodiment may turn off optical windows away
from the eyes of the observer so that as the observer 99 moves the
appearance of a display device 100 with greatly enhanced viewing freedom
may be achieved. Optical windows, such as optical window 125 which may
approximately correspond to locations between the eyes for example, may
be turned off to improve the crosstalk of the display images. Low
crosstalk advantageously may increase the perceived quality of 3D
stereoscopic images.

[0125] Further the observer location in two or three dimensions and motion
characteristics, such as velocity, acceleration, direction, and head
orientation may be determined from the sensor 70 and control unit 72.
This in turn may be used to generate the likely observer eye locations in
a future illumination time slot. Thus the appropriate illuminated
structure of the array 121 of optical windows may be determined to
optimize the output directionality of light from the display 100 in a
given illumination time slot, and may be determined by setting the
illumination structure of the respective illuminator elements of
illuminator array 15 for the time slot. Further, the image data on the
SLM 48 may be adjusted to advantageously achieve a look-around function,
a two dimensional image or other image characteristics as described
herein.

[0126] FIG. 16 is a schematic diagram illustrating the appearance of
further viewing windows in the window plane during observer movement. In
comparison to FIG. 15, optical windows between the eyes of the observer
99 may be illuminated for all observer positions. During tracking of a
moving observer, optical window 127 may change from a left eye optical
window to a right eye optical window. Advantageously increasing the
number of illuminated optical windows between the eyes of the observer
may reduce the flicker of images seen by the moving observer 99,
particularly for regions at the edge of the illuminated SLM 48 and for
observer positions away from the window plane 106. The appearance of
image flicker may be a more noticeable image degradation artefact than
the improvement in cross talk that may be achieved by non-illuminated
interaxial optical windows 125.

[0127] In the temporally multiplexed embodiments of the present
disclosure, as shown in FIG. 16, the optical window 127 changes from a
left eye phase to a right eye phase. Such a change may create undesirable
flicker as will be described.

[0128] There will now be described aspects of the control effected by the
control system described above that may be implemented in the display
apparatuses described herein.

[0129] FIG. 17 is a schematic diagram illustrating the timing of
illumination phases and observer location updates in the control system
under particular conditions. In particular, FIG. 17 shows the form of
control signals supplied by the control system to the illuminator
elements, taking the following form. Illumination pulses 3001 are
supplied during a left image phase to the illuminator elements selected
to direct light into a left eye viewing window. Similarly, illumination
pulses 3002 are supplied during a right image phase to the illuminator
elements selected to direct light into a right eye viewing window. The
left image phase and right image phase may have equal periods.

[0130] Further, FIG. 17 shows an embodiment where the display device may
be provided with, for example, an SLM that is an LCD running at 120 Hz
with alternating left and right eye image views. In this embodiment for a
given illuminator element of the array 15, the illumination pulses 3001,
3002 on time axis 3000 may be shorter than the full field time 3003 of
the display to illuminate substantially the whole display when the LCD
panel is substantially fully responded. In this embodiment it may be
understood that the location of the observer's left or right eye may be
employed at the time of the corresponding illumination pulse. Thus, the
steering system 74 in FIG. 12 may illuminate the observer's eye while
minimizing flicker and crosstalk. It may also be understood that the
tracking system sensor 70 in FIG. 12 may be embodied as a video camera
that may produce image frame updates 3008, 3010 separated in time 3004 in
time axis 3012, and at a rate less than the display field rate 3003. In
this case the controller 72 in FIG. 12 may use previous locations and the
time of those locations in order to determine the motion parameters of
the observer such as velocity and acceleration. Furthermore this data may
be used to predict the location of the observer at future times.

[0131] As discussed herein, the full field time may be referred to as the
time interval between addressing of a pixel of the temporally multiplexed
spatial light modulator in a sequential addressing scheme. The
illuminator elements may be arranged to be illuminated in synchronization
with the addressing of the spatial light modulator so in normal operation
(other than for illuminator elements associated with change of
illumination phase), the full field time is for example the time between
respective switch on points for the respective illuminator elements in
adjacent illumination intervals. As normal in display devices, the field
time is selected to be sufficiently short to minimize flicker in normal
operation, due to the persistence of vision.

[0132] Advantageously this embodiment may provide observer locations at
more times than produced by the tracking sensor 70 itself.

[0133] Advantageously the predicted locations may be used in cooperation
with the tracking sensor and controller 70, 72 of FIG. 12, to localize
the image frame search area and thereby reduce the amount of data to be
processed or searched in order to determine the location of the observer
in the image frame from the sensor 70.

[0134] FIG. 18 is a schematic diagram illustrating the timing of
illumination phases and asynchronous observer location updates of the
control system under different conditions. Further, FIG. 18 shows an
embodiment in which the observer's location may be generated or
calculated at times 3005 and at one or preferably more times within any
display field time 3003.

[0135] Advantageously this may achieve an embodiment in which the tracking
system may update the observer location to the steering system frequently
so that when an illumination pulses 3001, 3002 occur there may be a
reduction in error compared to only using updates separated in time 3004.
It may be readily understood that the more frequent the updates 3005, the
lower the observer's location error at the illumination time 3001, 3002.
It may also be understood that the tracking and location generating
system may not need to be synchronized to the image display system 76 or
illumination 74. Note that generating may refer to generating or
calculating locations in time rather than in space.

[0136] FIG. 19 is a schematic diagram illustrating the timing of
illumination phases; and synchronous observer location updates of the
control system under other different conditions. Further, FIG. 19 differs
from the embodiment of FIG. 18 in that the location updates on time axis
3012 from the tracking system 70, 72 may be substantially synchronized to
the display addressing 76, and the steering and illumination system 74.
Advantageously this means that many less extrapolated locations 3006 on
time axis 3000 may be needed than in the embodiment FIG. 18 and that
observer location accuracy error may be reduced. Advantageously the whole
time period 3007 is available for the tracking system 70, 72 to provide a
generated location at time 3006.

[0137] In the embodiment of FIGS. 18 and 19 it may also be understood that
the control system 72 may compare the generated observer location for a
future time with the actual location subsequently determined at or near
that time in order to produce an error value. The magnitude of this error
value may be used by the system as a measure of the "degree of tracking
lock" and the system may take actions based on this value including but
not limited to changing the size or nature of the illumination in the
optical window array 121 in FIG. 15. The illumination in the optical
window array 121 may be switched in to 2D mode for example. Note that
this action may take place at any point in the viewable field, not just
at the extremes.

[0138] The sensor, for example a video camera, may be calibrated by
methods such as steering the light in an illumination window to a number
of fixed locations of a detector such as a photo detector or a human eye,
which is recognized by the camera. Alternatively the detector may be
moved and aligned with a number of fixed positions defined by different
illuminator element array patterns. In a further example, the camera
vision system may itself see the light bar image of the illuminator
element array on, for example, the face of the observer and adjust it to
fall over the correct eye.

[0139] It may be recognized by those skilled in the art that the output
coordinate value from the tracking system may employ a significant
processing time and that the coordinate value produced may have a certain
lag or latency. Stated differently, the time at which the coordinate is
produced may reflect the location that an observer was in a short time
previously. This lag or latency may affect the maximum speed an observer
may move at without introducing an error in the illuminator element
position that may be perceived as flicker.

[0140] Advantageously the previously sampled locations may be used to
determine the velocity and or acceleration of the observer and these
figures may be processed with a knowledge of the latency to more
accurately report the location of the observer at the current time 3005
as shown in FIG. 18 or at a future required time 3006 corresponding to
the display illumination phase as shown in FIG. 19. Advantageously the
system may generate a single eye location, or determine both eye
locations from a generated nose location and knowledge of the eye
separation. The eye separation may be determined for each particular
observer from the camera images.

[0141] In the above examples, the illumination pulses 3001, 3002 are a
control signal supplied to the illuminator elements to cause them to
output a pulse of light having a period and magnitude of luminous flux
that are dependent on the period and amplitude, respectively, of the
illumination pulses 3001, 3002. In all the examples, described above the
illumination pulses 3001, 3002 have the same period and amplitude and so
cause the light sources to output light with the same period and luminous
flux. As the field time is sufficiently short to minimize flicker due to
the persistence of vision, the intensity of light perceived by the
observer may be considered as being dependent on the time-average of the
luminous flux over the field time, or over a single phase. This is
appropriate to the extent that the field time is short enough to minimize
flicker.

[0142] In general, the control system may be arranged, so that when the
position of the viewing windows is static, individual illuminator
elements are operated over a single phase (a right image phase or a left
image phase) so that the time-average of luminous flux over the phase has
a predetermined value. As a result, the intensity of light perceived by
an observer remains substantially constant over time, thereby minimizing
flicker to the extent that the field time is short enough to minimize
flicker.

[0143] The illumination pulses 3001, 3002 described above are an example
of an embodiment in which such control is effected, as a result of the
illumination pulses 3001, 3002 having the same period and amplitude. The
predetermined value may be substantially the time-average of luminous
flux achieved by the individual illuminator elements for time periods
when no observer movement has been detected. Such a predetermined value
may be set to achieve the desired display luminance after propagation of
the light to viewing windows through the directional backlight and SLM
48, with the illuminator elements operating in synchronization with the
display of image data that may be provided on the temporally multiplexed
SLM 48.

[0144] There will now be described examples of techniques for changing the
form of the illumination pulses, and hence the luminous flux of the light
output by the illuminator elements, that may be applied in the control
system.

[0145] FIG. 20A is a schematic diagram illustrating pulse amplitude
modulation for gray scale. Further, FIG. 20A shows how illumination
pulses of the same period 1000 for different illuminator elements in the
illuminator array 15 which may have slightly different performance
characteristics may be arranged to be adjusted in amplitude to have
different amplitudes 1002, 1004 providing pulses of light having luminous
flux of correspondingly different magnitude. Thus the time-average of
luminous flux is varied correspondingly. This may be used to achieve
luminous flux matching, for example, by amplitude modulation.

[0146] FIG. 20B is a schematic diagram illustrating pulse width modulation
for gray scale. Further, FIG. 20B shows how illumination pulses of the
same amplitude 1002 for different illuminator elements in the illuminator
array 15 may be adjusted by pulse width to have different periods 1000,
1006 providing pulses of light having periods of correspondingly
different length. Thus the time-average of luminous flux is varied
correspondingly.

[0148] Advantageously these pulse amplitude and/or pulse width modulation
techniques may achieve a substantially uniform luminous flux optical
window array 121. Advantageously the varied performance characteristics
of the individual LEDs may be substantially matched. Further
advantageously such matching may be calibrated at the window plane 106
and performed periodically during the lifetime of the display device.
Further advantageously the matching may be achieved by means of the
observer locating system 70, for example, a camera.

[0149] FIG. 21A is a schematic diagram illustrating a full illumination
pulse suitable for a weak emitter. Further, FIG. 21A shows an LED pulse
of period 1008 for an element of illuminator array 15. Such a pulse may
be suitable for the weakest element of the illuminator array 15.

[0150] FIG. 21B is a schematic diagram illustrating a chopped illumination
pulse suitable for a higher power illuminator element. Further, FIG. 21B
shows how the pulse width modulation, for example, to achieve gray scale
illumination or to match luminous flux may take place within the pulse
rather than at an end. Chop 1010 may be made in the pulse of one of the
stronger illuminator elements of the illuminator array 15 which may
achieve substantially the same output luminous flux as for a weaker
illuminator element of FIG. 21A. Different chops may be used to match all
the stronger illuminator elements to the weakest illuminator element.
Advantageously this may achieve substantially the same start and end time
for most to all illuminator elements of the illuminator array 15.
Advantageously the display device may be more uniformly illuminated
during any SLM 48 response times.

[0151] FIG. 21C is a schematic diagram illustrating a shorter pulse
suitable for a higher power illuminator element. Further, FIG. 21C shows
the pulse chop 1010 at the beginning of the pulse.

[0152] FIG. 21D is a schematic diagram illustrating a shorter pulse
suitable for a higher power illuminator element. Further, FIG. 21D shows
the pulse chop 1010 at the end of the pulse.

[0153] FIG. 22 is a schematic diagram illustrating a phased pulse position
modulation. Further, FIG. 22 shows how luminous flux matching may take
place using a pulse position modulation scheme. Each illumination pulse
may include a comb of individual illumination sub-pulses 1012. Further
the phase of the sub illumination pulses may be arranged to be
substantially "orthogonal" that is, to occur, for at least some, light
emitting elements at different times as shown by the two sets of
sub-pulses 1012 and 1014. Advantageously this may reduce the peak current
load on the illuminator power supply (not shown in FIG. 22) and therefore
reduce costs.

[0154] In the above embodiments image control unit 76 may use the observer
location data from sensor 70 and control unit 72 to achieve an image
display that varies in response to the observer 99 location.
Advantageously this may be used to provide a "look around" facility in
which, for example, the image perspective displayed on SLM 48 may be
varied in response to movement of the observer 99. Such movement may be
amplified to produce deliberately false perspectives.

[0155] In an illustrative embodiment in which the SLM 48 uses a liquid
crystal material, and is line by line addressed, the electro optic
response characteristics of the LC material may be important. Furthermore
the pulsed illumination may interact with the scanning and the LC
response in such a way that may result in different appearances of pixels
located at different spatial positions on the SLM 48 even if they were
addressed with the same original data. This effect may be eliminated by
pre-processing the raw image data to make a correction. A modification of
image data may also be made to compensate for predicated crosstalk
between left and right views.

[0156] Further advantageously the knowledge of the observer 99 location
may be used to provide a more effective adjustment of the image data to
SLM 48 in order to compensate for the effects described above.

[0157] In a further embodiment as shown in FIG. 23 the illuminator array
15 of light emitting elements may be aligned with an illuminator array of
emitting elements 515, for example infra-red LEDs, not intended for
visual illumination but which may be arranged to cooperate with sensor 70
to aid the detection of the observer 99 location. FIG. 23 is a schematic
diagram illustrating a further embodiment including an integrated
infrared emitting element array arranged to provide a fan of beams to
illuminate an observer and to provide an observer tracking function. The
infrared LEDs may provide an easily identifiable location of the
observer's pupils 599 through reflection from the retina. Thus element
514 may be directed to a window 526 approximately aligned with window 26
by the same optical system as for the display illumination, and thus be
substantially co-located in the viewing space. Advantageously, the LEDs
may be arranged in the same package to reduce the cost of alignment and
packaging of the respective illuminator array 515 and light emitting
element illuminator array 15. Advantageously the illuminator array of
emitting elements 515 may be separately addressable from the illuminator
array 15. Advantageously such an arrangement may achieve a self-aligning
fan of infrared beams that may provide illumination of the observer
plane. The fan of beams may be temporally scanned in luminous flux so the
timing of the retro reflected beam may be measured, achieving a low cost
measurement system compared to the camera sensor and processing overhead
of the camera embodiments. Alternatively, a separate infrared detector
may be used to detect pupil position.

[0158] There will now be considered the operation of the control system
when shifting the position of the viewing windows in response to the
detected position of the observer changing laterally across the display
device 100. In this case the control system controls the illuminator
elements corresponding to optical windows of left and right viewing
windows that are closest to each other by ceasing operation of a given
illuminator element in one of the left and right image phases and
starting operation of the same or different illuminator element in the
other one of the left and right image phases.

[0159] FIG. 24 is a schematic diagram illustrating an example of pulse
waveforms 5002, 5004 that may be supplied to at least one light emitting
element illuminator array 15 that may be provided during operation in an
autostereoscopic display apparatus. An example waveform suitable for
driving the illuminator elements, which in this illustrative embodiment
are LEDs, in the left image phase corresponding to illumination of a left
optical window is 5002 and an example waveform for the right image phase
corresponding to the right optical window is 5004. Thus the dotted lines
represent a time interval 5021 between the start of subsequent left image
phases, corresponding to a left image phase and a right image phase
combined.

[0160] In this example and the subsequent examples described herein,
operation is ceased in the left image phase and started in the right
image phase. This is performed in response to detection of the observer
99 moving from right to left across the display device 100. An inverse
control, that is ceasing operation in the right image phase and starting
operation in the right image phase is performed in response to detection
of the observer 99 moving from left to right across the display device
100. The control in response to different lateral movement of the
observer 99 is entirely symmetrical and so this and subsequent examples
may equally be applied to movement of the observer in the opposite
direction by inverting the right and left image phases. Thus, the inverse
control is not separately described or illustrated. However, references
(a) left and (b) right, may be generalized to (a) either one of left or
right, and (b) the other one of left or right.

[0161] In general, depending on whether the left and right viewing windows
are separated, the waveforms 5002, 5004 may be applied to the same
illuminator elements or to different illuminator elements. That is, if
there is no separation between the left and right viewing windows, then
the waveforms 5002, 5004 may be applied to the same illuminator element,
so that an illuminator element in the nose region of the observer 99
ceases operation in the left image phase and starts operation in the
right image phase. Conversely, if there is a separation between the left
and right viewing windows, then the waveforms 5002, 5004 may be applied
to different illuminator elements, so that an illuminator element to the
trailing side of the nose region of the observer 99 ceases operation in
the left image phase and another illuminator element to the leading side
of the nose region of the observer 99 starts operation in the right image
phase. However, for ease of understanding, the pulses in the left and
right image phases are separated as the two waveforms 5002, 5004 in FIG.
24 irrespective of whether they are applied to the same illuminator
element.

[0162] The time interval 5021 is determined by the update rate of the left
image to the SLM 48. The timing of pulses 5006, 5012 in the waveforms
5002, 5004 is phase shifted and is arranged in correspondence to the
timing of the display of left image and right image data on the SLM 48.
The length of the ON, or illumination, pulse 5006 may be less than the
respective field length of the SLM 48, to achieve improved cross talk and
contrast of the displayed image. In operation at least one group of
illuminator elements is directed to the left eye, driven by for example
waveform 5002 and at least one group of illuminator elements is directed
to the right eye, driven by for example waveform 5004. It is advantageous
to give special treatment to the boundary between the left and right eye
groups, for example as will be described below.

[0163] A stationary observer may thus receive light from the respective
illuminator elements of illuminator array 15 illuminated by pulses 5006
in the left image phase. When the observer moves position across the
window 26 such that the right eye moves toward the original position of
the left eye, then the control system (described above) may determine
that the window 26 illuminated by the respective illuminator element
should be substantially synchronized with the right eye image rather than
the left eye image. As discussed herein, pulses such as 5006, 5012 of
pulse waveforms 5002, 5004 respectively of FIG. 24 may be referred to as
pulses of left and right image phases respectively. Thus the left image
phase of the SLM 48 is synchronized with the left pulses 5006 and the
right image phase of the SLM 48 is synchronized with the right pulses
5012. The time interval of the transition region may be the same as the
time interval 5021, and may be determined by the update timing of the SLM
48.

[0164] Accordingly, the respective illuminator element may change
illumination phase from synchronization with the left image to
synchronization with the right image. The light may thus steer to the
correct position for the moving observer (as described above) with the
left to right transition in the region of the observer's nose. The
illumination waveform is illustrated in FIG. 24 as solid lines for left
and right phases of illumination, with dotted lines illustrating what the
illumination phase would have been without switching phase. Thus the last
left pulse starts at time 5014 whereas the first right pulse starts at
time 5016.

[0165] FIG. 24 illustrate an example in which the final instance of
operation in the left image phase and the initial instance of operation
in the right image phase occur in an adjacent pair of left and right
image phases. Also illustrated in FIG. 24 is a waveform 5018 representing
the intensity of light perceived by the observer 99 resulting from
application of the waveforms 5002, 5004 (noting that when the waveforms
5002, 5004 are applied to the same illuminator element, this corresponds
to the waveform applied thereto) as the observer 99 moves to one side. It
is noted that this waveform shows a transition from illumination in the
left phase to illumination in the right phase, and at the transition
region 5020 a closely spaced or double pulse 5022 may occur. The waveform
5018 in FIG. 24 may result in a perceived brightness artifact in the
transition region 5020 that may be seen by the observer 99 as a bright
flash.

[0166] Conversely, FIG. 25 illustrates an example similar to that of FIG.
24 but in which the initial instance of operation in the right image
phase is delayed compared to FIG. 24 to the next available right image
phase with the result that there is an adjacent pair of left and right
image phases in time period 5023 in which there are no pulses at all.
Thus, in the transition region 5020 a widely spaced or missing pulse may
occur. In this case, in the transition region 5020 with a time interval
5025 that may be the same as time interval 5021, a pulse that would have
occurred at time 5024 does not occur, and instead the next pulse appears
at time 5026. Thus, the waveform 5018 in FIG. 25 may result in a
perceived brightness artifact in the transition region 5020 that may be
seen by the observer 99 as a dark flash.

[0167] Therefore, whichever of the alternative waveforms of FIGS. 24 and
25 are applied, the observer may perceive a brightness artifact in the
transition region 5020 that may be seen as either a bright or a dark
flash.

[0168] There will now be described and illustrated some embodiments in
which the control performed by the control system is modified to reduce
such brightness artifacts.

[0169] In FIGS. 26 to 31, there are illustrated some embodiments wherein
the illuminator elements operated in the left image phases are contiguous
in the array of illuminator elements with the illuminator elements
operated in the right image phases so that the same illuminator element
ceases operation in the left image phase and starts operation in the
right image phase. Thus, FIGS. 26 to 31 illustrate waveforms of the
control signals supplied to a given illuminator element to cause it to
output light, consisting of illumination pulses in left and right image
phases, as described above.

[0170] FIG. 26 illustrates schematically one example of compensation for a
double pulse (bright) artifact. The control signal supplied to the
illuminator element has a waveform 5030 that in region 5020 causes
operation in the left image phase to cease and operation in the right
image phase to start. In this example, similar to waveform 5018 shown in
FIG. 24, the final instance of operation in the left image phase is
before the initial instance of operation in the right image phase by
pulse 5032, and the final instance of operation in the left image phase
is by a pulse that has a normal pulse period so that over that phase the
time-average of luminous flux is the predetermined value. However, in
contrast with waveform 5018 shown in FIG. 24, the initial instance of
operation in the right image phase is by pulse 5032 that has a shortened
period so that over that phase the time-average of luminous flux is less
than the predetermined value.

[0171] Advantageously, in the present embodiments, by processing the
waveforms to the illuminator elements of the illuminator array 15 in the
transition regions between left and right phases the conditions that may
result in a brightness artifact can be compensated for.

[0172] FIG. 27 illustrates schematically a further example of compensation
for a double pulse (bright) artifact. The control signal supplied to the
illuminator element has a waveform 5035 that in region 5020 causes
operation in the left image phase to cease and operation in the right
image phase to start. In this example, similar to waveform 5018 shown in
FIG. 24, the final instance of operation in the left image phase by a
pulse 5033 is before the initial instance of operation in the right image
phase by a pulse which itself has a normal pulse period so that over that
phase the time-average of luminous flux is the predetermined value.
However, in contrast with waveform 5018 shown in FIG. 24, the final
instance of operation in the left image phase is by a pulse 5033 that
itself has a shortened period so that over that phase the time-average of
luminous flux is less than the predetermined value. Thus, the pulse 5033
may be arranged to be in the opposite image phase to the pulse 5032 of
FIG. 26, so that for example a right image is seen rather than a left
image at the time of transition. Advantageously, such an embodiment may
achieve reduced image flicker.

[0173] FIG. 28 illustrates schematically one example of compensation for a
missing (dark) pulse in original waveform 5018 in region 5020, as
illustrated in FIG. 25. The control signal supplied to the illuminator
element has a waveform 5031 that in region 5028 causes operation in the
left image phase to cease and operation in the right image phase to
start. In this example, the waveform 5031 is similar to the waveform 5035
of FIG. 27.

[0174] FIG. 29 is a schematic diagram illustrating a further embodiment
including centering of the intermediate pulse 5040. The control signal
supplied to the illuminator element has a waveform 5036 that in region
5038 causes operation in the left image phase to cease and operation in
the right image phase to start. In this example, the waveform 5036 is
similar to the waveform 5030 of FIG. 26, except that the initial instance
of operation in the right image phase a pulse 5040 that is shifted so as
to be arranged temporally approximately equidistant with the same time
interval 5042 between preceding and following pulses as illustrated in
FIG. 29. Advantageously such an embodiment achieves further reduction in
appearance of the flicker artifact for a moving observer.

[0175] FIG. 30 is a schematic diagram illustrating a further embodiment
including arranging of the intermediate pulse 5040 around the start of an
illumination phase. The control signal supplied to the illuminator
element has a waveform 5037 that in region 5038 causes operation in the
left image phase to cease and operation in the right image phase to
start. In this example, the intermediate pulse 5040 is effectively formed
by the initial instance of operation in the right image phase and the
final instance of operation in the left image phase. Thus, the final
instance of operation in the left image phase is after the initial
instance of operation in the right image phase. Furthermore, the final
instance of operation in the left image phase and the initial instance of
operation in the right image phase are each performed by pulses having a
shortened period that is half the normal period so that over each
respective phase the time-average of luminous flux is half the
predetermined value. Advantageously, modifying the drive waveforms as
described in FIG. 30 may achieve a mixture of left and right eye
illumination of the SLM 48 at the transition, and thus reduce flicker
artifacts in those parts of the display from which light from the
transition illuminator elements are visible.

[0176] In all of the above examples, the brightness artifacts of a bright
flash of the type of FIG. 24 or a dark flash of the type of FIG. 25 are
reduced by altering the waveform of the control signal in a manner that
reduces the overall time-average of the luminous flux at the in region
5020 compared to FIG. 24 and that increases the overall time-average of
the luminous flux at the in region 5020 compared to FIG. 25. Other
waveforms may be used to produce a similar effect.

[0177] The situation shown in FIG. 24 arises because the transition causes
an adjacent pair of a left image phase and a right image phase to have a
time-average of luminous flux of the illuminator element that is twice
said predetermined value. In the general case to reduce flicker, the
control signal is modified so that over each adjacent pair of a left
image phase and a right image phase, the time-average of the luminous
flux of the illuminator element which is changed from operation between
the left and right image phases is less than twice said predetermined
value. This is achieved by each of the waveforms shown in FIGS. 26 to 30.

[0178] Similarly, the situation shown in FIG. 25 arises because the
transition causes an adjacent pair of a left image phase and a right
image phase to have a time-average of luminous flux of the illuminator
element that is zero. In the general case to reduce flicker, the control
signal is modified so that over each adjacent pair of a left image phase
and a right image phase, the time-average of the luminous flux of the
illuminator element which is changed from operation between the left and
right image phases is more than zero. This is achieved by each of the
waveforms shown in FIGS. 26 to 30.

[0179] In all of the above examples, the time-average of luminous flux of
a pulse is controlled to be less than the predetermined value by reducing
the period of the pulse, that is by time modulation. In general, such
control of the time-average of luminous flux of a pulse may be performed
using any one or any combination of the techniques shown in FIGS. 20A to
21D, for example by changing the amplitude, position and/or duration of
the pulse 5032.

[0180] FIG. 31 is a schematic diagram illustrating a further embodiment
including providing increased luminous flux of pulses in the transition
region 5020. The control signal supplied to the illuminator element has a
waveform 5018 that in region 5020 causes operation in the left image
phase to cease and operation in the right image phase to start. In this
example, the waveform 5018 is the same as shown in FIG. 25 except that
the final instance of operation in the left image phase and the initial
instance of operation in the right image phase are performed,
respectively, by pulses 5043 and 5045 that have an increased amplitude so
that over that over each of those phases the time-average of luminous
flux is greater than the predetermined value. Thus the time integrated
luminous flux over the transition region 5020 is matched to the time
integrated luminous flux at other times, thereby reducing the brightness
artifact of a dark flash that occurs in FIG. 25. Advantageously,
modifying the drive waveforms as described in FIG. 31 may achieve reduced
flicker artifacts in those parts of the display from which light from the
transition illuminator elements are visible.

[0181] Advantageously, modifying the drive waveforms as described in FIGS.
26 to 31 may reduce the appearance of a brightness flicker effect for the
observer and thus improve the quality of the display for a tracked
observer.

[0182] Such modification of the control may be implemented in the control
system in a straightforward manner simply by modifying the form of the
control signals generated thereby. Some possible techniques are as
follows.

[0183] FIG. 32 is a schematic diagram illustrating a further embodiment
comprising a further control method to drive a switch illuminator element
with a modified output to reduce image flicker.

[0184] The first step 500 is to identify the candidate illuminator
elements which may potentially be affected by a bright or dark pulse
artifact. These illuminator elements correspond to those in a region
which is imaged physically to the region between the eyes corresponding
to the nose position of the observer. This may be done for example as
will be described in FIG. 43.

[0185] The next step 502 is to determine if the correction is required for
a bright pulse (a short gap between pulses) or a dark pulse (a long gap
between pulses). This may be achieved for example by the method
illustrated in FIGS. 44 and 60C. Other detection methods such as timing
or measuring the pulse gap for example with a counter/timer circuit could
be used. In the case of a bright pulse event block 506 is used to reduce
the pulse that is applied to the affected illuminator elements. The pulse
may be reduced in length for example to 50% of the width of the pulse
that was previously applied or may be reduced in amplitude or a
combination of the two so that the effect of the bright pulse artifact is
reduced.

[0186] In the case of a dark pulse artifact, block 504 is used to insert a
short pulse for example a pulse 50% of the length of the standard
illumination pulse width. The pulse may alternatively be reduced in
amplitude or a combination of width and amplitude in order to compensate
for the dark pulse artifact. The modified illuminator element pulse
information is then passed to block 508 which controls drive to the
illuminator element array.

[0187] FIG. 33 is a schematic diagram illustrating a further embodiment
including switching of an illuminator element array in a first pattern
between first and second illumination positions. illuminator array 400
comprising individually addressable array of illuminator elements is
arranged with non-illuminated illuminator elements 406, left phase
illuminated illuminator elements 402 and right phase illuminated
illuminator elements 404. After an observer movement, illuminator
elements 410 and 412 are used instead of 402, 404 respectively. As
described in FIGS. 24 and 25, such switching and position 414 can create
a flicker artifact. Similar but typically less detrimental artifact can
be observed for illuminator element positions 416, 418. However, such
positions are typically removed from eye positions compared to position
414 and are of less significance. However, the correction methods
described herein can advantageously be applied to correct luminous flux
changes in window positions corresponding to illuminator elements 416,
418 if desired.

[0188] FIG. 34 is a schematic diagram illustrating a further embodiment
including switching of an illuminator array in a second pattern between
first and second illumination positions. In this embodiment, a central
illuminator element 420 is not illuminated. Advantageously, such an
arrangement can improve image cross talk. However, light from positions
424, 426 can bleed to both left and right eyes. Such a bleeding can
result in a flicker artifact similar to that described in FIGS. 24 and
25, even though two illuminator elements are switching. Thus the
correction mechanisms of the present embodiments can advantageously
applied to arrangements with non-contiguous left and right LEDs 402, 404.

[0189] FIG. 35 is a schematic diagram illustrating a further embodiment
comprising a control system to achieve uniform switching of an
illuminator array between first and second positions. Illumination
controller 74 may comprise data input 440 containing position information
about the Observer, information controller 428 to determine illuminator
element left and right phase control lines 442, 444 based on the
illumination mode the display is operating in, transition illuminator
element controller 430 which identifies potential flicker artifacts and
compensates for them, and may further comprise illuminator element driver
432 arranged to drive lines 446 connected to illuminator elements of
array 15. Alternatively transition illuminator element control may be
performed with the control lines 446 rather than as a separate
pre-conditioning control element. Advantageously such a control system
can achieve real time correction of transition illuminator element
switching, thus reducing image flicker at low cost and high speed.

[0190] It is desirable to identify which groups of illuminator elements
may be switching from left to right eye phases, thus creating flicker
artifact identified in FIGS. 24 and 25. Such illuminator element groups
may be those in the nose region, or in other regions where the phase of
an illuminator element or closely spaced illuminator elements changes
from left to right eye phase. Such illuminator elements may be referred
to herein as transition illuminator elements. Note they need not be at
the centre of the illuminator array 15, and the transition illuminator
elements move with the position of the observer.

[0191] FIG. 36 is a schematic diagram illustrating the steps of switching
of an illuminator array in a second pattern between first and second
illumination positions with no central illuminator element control. In a
similar manner to that shown in FIGS. 24 and 25, it can be seen that in a
first switching sequence, the left eye group 402 moves before the right
eye group 404 so that group 403 is arranged contiguous with group 404.
This removes the gap between the windows for a brief period before group
404 moves to group 412. In this manner, the corresponding window will see
a pulse of higher brightness.

[0192] FIG. 37 is a schematic diagram illustrating the steps of switching
of an illuminator array in a second pattern between first and second
illumination positions with no central illuminator element control. Thus
in a second switching sequence, the left eye group 402 moves after the
right eye group 404so that group 405 is arranged with a group 421 of no
illumination. This doubles the gap between the windows for a brief period
before group 402 moves to group 410. In this manner, the corresponding
window will see a pulse of lower brightness.

[0193] Thus it can be shown that the control of switching of the central
illuminator elements is required for both contiguous and non-contiguous
groups 402, 404 of illuminator elements.

[0194] Whilst FIGS. 26 to 31 illustrate embodiments wherein the same
illuminator element starts operation in the right image phase as ceased
operation in the left image phase, as an alternative a different
illuminator element may start operation in the right image phase as
ceased operation in the left image phase. This occurs in the case that
the illuminator elements operated in the left and right image phases are
separated in the array of illuminator a different. With such control the
reduction of brightness artifacts may be achieved in a similar manner to
that described above, except that the illuminator element that ceases
operation in the left image phase and the illuminator that starts
operation in the right image phase are controlled by separate control
signals, rather than a common control signal. Embodiments equivalent to
those of FIGS. 38 to 43 may be implemented by splitting the control
signals shown therein into such separate control signals. Some other
examples of embodiments where two different illuminator elements start
operation in the right image phase and cease operation in the left image
phase are as follows.

[0195] FIGS. 38 and 39 are schematic diagrams illustrating pulse waveforms
420, 422 of control signals applied to the different illuminator elements
that are equivalent to the control signals shown in FIGS. 24 and 25, and
which would respectively produce bright and dark flicker artifacts during
switching of a illuminator element between left and right image phases.

[0196] In contrast, FIGS. 40 and 41 are schematic diagrams illustrating
pulse waveforms 420, 422 of control signals applied to the different
illuminator elements that reduce such brightness artifacts by
respectively introducing pulses 5047, 5049 that have reduced period to
reduce the visibility of the brightness artifact but maintain panel
illumination.

[0197] In the case of FIG. 40, the additional pulse 5047 is provided in
the control signal having waveform 420 to perform the final instance of
operation in the left image phase. The result is similar to that of FIG.
27, as follows. The final instance of operation in the left image phase
by a pulse 5047 provided by the control signal of waveform 420 is before
the initial instance of operation in the right image phase provided by
the control signal of waveform 422 supplied to the different illuminator
element. That initial instance of operation in the right image phase is
performed by a pulse in waveform 422 that itself has a normal pulse
period so that over that phase the time-average of luminous flux is the
predetermined value. However, the final instance of operation in the left
image phase by a pulse 5047 in waveform 420 that has a shortened period
so that over that phase the time-average of luminous flux is less than
the predetermined value.

[0198] FIG. 42 is a schematic diagram illustrating further pulse waveforms
420, 422 wherein a pulse 5051 is inserted comprising more than one
illumination time, with a total width the same as pulse 5049 of FIG. 42.
Advantageously, the inserted illumination pulse can be arranged to
coincide with illumination of more than the top region of a sequentially
addressed spatial light modulator, increasing uniformity across the area
of the SLM 48. FIG. 43 is a schematic diagram illustrating a control
method that may optionally be implemented in the control system to
identify transition LEDs. Reference is made to an illuminator array
comprising LEDs, but similar techniques could be applied to other types
of light source.

[0199] It would be desirable to implement the control of the transition
LEDs locally to the LED drive system as opposed to within a system
controller, thus reducing cost and complexity. FIG. 43 describes a method
to achieve local control of the transition LEDs, that is the LEDs in the
transition region 5020 at a given time. The transition LEDs may be
identified in the illumination array control circuit by for example the
following method. Arrays 6050 and 6052 comprise groups 6060 and 6062
corresponding to illuminated LEDs in a first position for example and
corresponding to the illumination of the left and right eyes
respectively. In a first step, the group 6060 in array 6054 is shifted to
the right to achieve array 6054 with group 6064, while group 6062 is
shifted to the left in array 6052 to achieve array 6056 with group 6066.
In a second step a logical AND operation may be performed between arrays
6054, 6056 to create array 6070 with group 6072 that identifies the
transition LEDs and to which a correction for a flicker artifact may be
advantageous.

[0200] In an arrangement as shown in FIG. 8 for example, LED arrays may
provide multiple groups of illuminated LEDs. It may be desirable to
identify respective switching LED patterns between the multiple groups of
LEDs that switch in a similar manner to the transition LEDs.
Advantageously flicker for regions of the LED array that are switching
between left and right eye phases for moving observers can be reduced.

[0201] FIG. 44 is a schematic diagram illustrating a further control
method to identify switching LED groups in a first illumination
arrangement corresponding to a bright pulse artifact. FIG. 45 is a
schematic diagram illustrating a further control method to identify
switching LED groups corresponding to a dark pulse artifact. Which of the
transition LEDs benefits from a correction may be identified in the
illumination array control circuit by for example the following method
illustrated in FIGS. 44 and 45. The last illumination array 6080 with
right eye illumination group 6100 applied to the light emitting element
array 15 (such as an LED array) and the proposed next array 6082 with
left eye group 6102 are logical XNOR (exclusive NOR) together to produce
array 6084 which identifies the LED group 6085 which did not change state
between left and right illumination phases. It will be apparent that many
of the LEDs that do not change state were not illuminated in either
phase, and these may be identified by logical ANDing the array 6084 with
the array 6070 which identified the transition LED group 6072 by the
method described with reference to FIG. 43.

[0202] Array 6088 contains LED 6108 to which a correction can be applied
to reduce flicker artifact. Looking at array 6080, the original state of
LED 6108 was ON, then a low brightness correction pulse (e.g. reduced
width pulse) may be appropriate to correct a bright flash artifact. This
is applied with reference to control flow of FIG. 30 and circuit blocks
of FIG. 35. Similarly if the array 6090 with right eye illumination group
6200 is XNORed with the proposed next array 6092 with left eye
illumination group 6202 to give the array 6094 representing the LEDs that
do not change state, and this group is logical ANDed with the transition
LED group 6072 of array 6070, the resulting array 6098 contains the LED
(in this case 6110) which benefit from an applied correction. As the
original state of this LED in array 6090 was OFF, then an extra pulse is
needed to correct for a dark flash artifact.

[0203] Implementation for correction of the artifact illustrated in FIGS.
36 and 37 may be achieved by inserting a data shift of the groups 402,
404 prior to characterizing the switched LEDs. In this manner the same
process can be achieved, with a gap reinserted after correction.

[0204] Advantageously the present embodiments are readily compatible with
implementation by circuitry including but not limited to ASICs and FPGAs,
for example implementation of the flow diagram of FIG. 30.

[0205] The arrangements of FIGS. 24-45 may be applied to temporally
multiplexed displays in which a illuminator element is arranged to change
phase to switch from left to right window imagery. Such displays may
include, but are not limited to optical valve directional backlights,
Fresnel lens, microlens array, wedge type illumination systems, and so
forth.

[0206] As may be used herein, the terms "substantially" and
"approximately" provide an industry-accepted tolerance for its
corresponding term and/or relativity between items. Such an
industry-accepted tolerance ranges from zero percent to ten percent and
corresponds to, but is not limited to, component values, angles, et
cetera. Such relativity between items ranges between approximately zero
percent to ten percent.

[0207] While various embodiments in accordance with the principles
disclosed herein have been described above, it should be understood that
they have been presented by way of example only, and not limitation.
Thus, the breadth and scope of this disclosure should not be limited by
any of the above-described exemplary embodiments, but should be defined
only in accordance with any claims and their equivalents issuing from
this disclosure. Furthermore, the above advantages and features are
provided in described embodiments, but shall not limit the application of
such issued claims to processes and structures accomplishing any or all
of the above advantages.

[0208] Additionally, the section headings herein are provided for
consistency with the suggestions under 37 CFR 1.77 or otherwise to
provide organizational cues. These headings shall not limit or
characterize the embodiment(s) set out in any claims that may issue from
this disclosure. Specifically and by way of example, although the
headings refer to a "Technical Field," the claims should not be limited
by the language chosen under this heading to describe the so-called
field. Further, a description of a technology in the "Background" is not
to be construed as an admission that certain technology is prior art to
any embodiment(s) in this disclosure. Neither is the "Summary" to be
considered as a characterization of the embodiment(s) set forth in issued
claims. Furthermore, any reference in this disclosure to "invention" in
the singular should not be used to argue that there is only a single
point of novelty in this disclosure. Multiple embodiments may be set
forth according to the limitations of the multiple claims issuing from
this disclosure, and such claims accordingly define the embodiment(s),
and their equivalents, that are protected thereby. In all instances, the
scope of such claims shall be considered on their own merits in light of
this disclosure, but should not be constrained by the headings set forth
herein.